1. Field of the Invention
The invention relates to an improved method and apparatus for use in pulse echo distance measurement.
2. Description of the Related Art
Pulse echo methods are well known for use in distance measuring. In such methods a pulse of radiation is transmitted towards a target, reflected at the target, and received by a sensor and the time elapsed between transmission and the time when the first or primary echo is received from the target is measured. Multiplying this travel time by the pulse propagation velocity gives twice the distance to the target. Types of radiation commonly used are electromagnetic radiation (radar), optical radiation (lidar) or acoustics (sonar). The target may be a solid object whose position is to be pinpointed or the target may be the boundary between a liquid and a gas or between two or more liquids to measure the depth of one or more of the liquids. The liquid may be flowing within a channel, for example, a river or may be non-flowing, in a natural basin or within a tank. Typically in such systems, the signal is an ultrasonic signal using a submerged ultrasonic sensor or the sensor may be fixed to a rigid structure in air above a target surface, looking down on the target. In a further prior art arrangement, a fixed submerged sensor is mounted to a structure such as a bridge and utilized to monitor a distance from the sensor to the surface of, for example, a river bed. Variations in distance imply that either the bridge is shifting or that the bridge foundations are being eroded.
Such prior art devices determine the value relating to the primary echo. This is achieved by measuring the time elapsed between applying an electronic stimulus to the transmitting element and the detection of the first reflected electronic echo signal from the receiving element. This measurement contains delays not related to the pulse travel time through the liquid due to the delay between the stimulus to the transmitting element and the actual transmission of a signal and the time travelled within the sensor until it reaches its surface before being directed towards the target. Similarly there is a delay between the signal reaching the sensor surface and travelling to the receiving element and then the detection of the echo signal from the receiving element. These delays which may vary with operating conditions have to be removed by calibration. This can lead to either expensive calibration or errors in the system. The primary echo can be confused by spurious echoes from materials within the liquid, by missing echoes and when the primary echo falls within the blanking period (to be discussed later). This can cause unnecessary errors.